The present invention relates to nuclear power plants, and more particularly, to the automatic response of the nuclear reactor control system, to a sudden and unexpected loss of function of a major plant component.
The nuclear steam supply system (NSSS) of a pressurized water nuclear power plant, normally operates with minor perturbations in power and flow. These can be handled by the basic reactor control systems. Certain large plant imbalances can occur however, such as a large turbine load rejection, turbine trip or partial loss of on-line main feedwater pump capacity. Under these conditions, maintaining the NSSS within the control band ranges can be accomplished by rapid reduction of NSSS power at a rate which is greater than that provided by the normal high speed control rod insertion. In addition, rapid NSSS power reduction is effective in gaining sufficient thermal margin to accommodate inward control rod deviations (including spurious rod drops) without a reactor trip.
One known system for achieving this objective, is commercially available as the Reactor Power Cutback System (RPCS) from Combustion Engineering, Inc., Windsor, Conn. U.S. Pat. No. 4,075,059, issued on Feb. 21, 1978, for "Reactor Power Reduction System and Method", describes such RPCS. The disclosure of this patent is hereby incorporated by reference. The system is designed to accommodate large load rejections, the loss of one feedwater pump or inward deviations of control rods (including spurious rod drops), by providing a "step" reduction in reactor power. The step reduction in reactor power is accomplished by the simultaneous dropping of one or more preselected groups of full strength regulating control rods into the core. The control rod groups are dropped in their normal sequence of insertion. The RPCS also provides control signals to the turbine to rebalance turbine and reactor power following the initial reduction in reactor power as well as to restore steam generator water level and pressure to their normal controlled values.
Conventional NSSS have two steam generators and two variable speed main feedwater pumps which deliver secondary coolant water to a common conduit which divides and is controlled by a respective valve to each steam generator. On the occurrence of a loss of a steam generator feedwater pump, the flow of the secondary coolant to the steam generator is reduced, since the remaining pump is unable to provide 100% of the feedwater demanded by the steam generators. When this happens, the steam generator consumes water (by converting it into steam) faster than the water in the steam generator can be replaced, which results in a consequent drop of the water level in the steam generator. This situation cannot be tolerated for very long before the reactor and turbine are caused to trip on a signal indicating low steam generator water level. If the reactor power is immediately and rapidly reduced to a sufficient degree, however, the steam generator's ability to produce steam is reduced, thereby permitting avoidance of the reactor-turbine trip on a low steam generator water level. In the typical nuclear power plant, where two feedwater pumps with 65% of full feedwater flow capacity to each steam generator are provided, the initial reduction in power demanded by the loss of one feedwater pump in order to avoid a reactor trip is 75% of full reactor power. This restricted power, which allows continued reactor operation after the loss of a feedwater pump, is determined by the capacity of the pumps of the system and the characteristics of the steam generator.
For plant start-up, the feedwater pumps are started manually by the plant operators, either by providing steam and cooling water to a turbine driver or by supplying electricity to a motor driver. The pumps draw water from, and recirculate it to, a feedwater or condensate storage tank. When the pumps have been warmed up and checked for proper operation, their speed is manually set at a minimum controlled speed by the operators. When plant conditions allow switching one or both of the feedwater pumps to automatic control by a feedwater control system, the operators switch the control mode of the selected pump(s) to automatic. In automatic control, the feedwater control system will set the pump speed, and control valve positions, to maintain a pre-set water level in the steam generators.
In these plants, the feedwater pumps and the RPCS are operated in the following manner:
______________________________________ Power RPCS Plant Level Feedwater Status Status Operator Actions Response ______________________________________ 0 to One FW pump Out of Prior to raising If the 50% ON (running at service power above 5% operating operating speed, place Feedwater feedwater supplying water Control System in pump trips, to the steam automatic. This reactor trip generators) and controls the FW will be second pump pump running at initiated on either ON but in operating speed. low level in standby (running After lowering the steam at minimum power below 5%, generator. recirculation place feedwater speed, not control in manual. supplying water to the steam generators), or OFF and out of service. 50% Both feedwater In service At approximately If one of the to pumps ON and 50% power main 100% running at (increasing), the feedwater operating speed. second feedwater pump trips, pump is placed in the reactor automatic control power from the control cutback will room. The RPCS be initiated is placed in by RPCS trip service when the demand second feedwater signal. pump is placed in automatic control. ______________________________________
The need has been identified for providing an NSSS with three main feedwater pumps; the third is to be used either as a spare or in a sharing mode with the other two. The conventional RPCS can easily be incorporated into an NSSS configuration with three main feedwater pumps, but the present inventors have conceived an improvement to the RPCS logic, which takes advantage of the greater operational flexibility afforded by a three pump NSSS.